Apple is including a predictive text model which offers suggestions while you type, which they’ve said to be a "transformer model". I managed to find some details about this model
Early on I thought the emoji would be associated with a clean classification (say a ringed planet for astronomy) but I found I wasn't really doing that (I used two people holding hands for a post about a possible companion for a black hole) From the very beginning I thought about making something that automatically suggests an emoji but I've yet to do anything about it: the existing models I have would treat it as a many-shot problem based on emoji I've used before but I've also thought about making something trained on definitions or plain text descriptions of the emoji and related concepts.